1. Field of the Invention
The present invention relates to an optical disc for recording, erasing or reproducing information using a laser beam and more particularly, to a protective coat for a substrate of optical disc which is made of plastics.
2. Description of the Related Art
As a substrate of an optical disc, a thermoplastic resin such as PC (polycarbonate) or PMMA (poly methyl- methacrylate) is widely used. Since such plastics substrate is a dielectric, static electricity is likely to be electrically charged, so that dust or the like is easily attached onto the substrate surface, causing a laser beam to scatter. Therefore, recording and reproducing are not normally performed. In order to solve such problem, there is provided a transparent antistatic resin film on the side of the substrate where the optical beam is applied, which is disclosed in Japanese Patent Publication No. 158643/1989.
As the transparent antistatic resin film, there have been known an antistatic resin film mixed with a surface active agent, a siloxane, an inorganic transparent electro-conductive compound, an electro-conductive filler or the like.
As for the antistatic resin film mixed with a surface active agent, as a ratio of the surface-active agent to the resin is increased, an antistatic effect is heighten but surface hardness is lowered. Contrarily, as that ratio is decreased, the surface hardness is improved but the anti-static effect is lowered, which means that the antistatic effect and the surface hardness can not be satisfied at the same time. In addition, when it is exposed to condition of the high temperature and humidity, for example at 80.degree. C. and 90%RH, the surface-active agent comes out on the surface of the film and then water is attached to that surface-active agent so that a waterdrop is generated, which is a problem in respect of long-term stability.
As for the antistatic resin film mixed with a siloxane, although the antistatic effect is high and the film is hard itself, since its film thickness can not be thick, that is, below 0.2 .mu.m, the sufficient surface hardness can not be obtained on a soft plastics substrate. When the optical disc comprising the antistatic resin film mixed with the siloxane is put to a low temperature and high temperature cycle test, a surface crack could be generated because adhesive property between the film and the substrate is not good. When it is exposed to the condition of high temperature, for example 80.degree. C., the antistatic effect is considerably lowered and the long-term stability is not enough.
In addition, as for the antistatic resin film mixed with an inorganic transparent electro-conductive compound such as ITO (In.sub.2 O.sub.3 : Sn) or tin oxide (SnO.sub.2), the adhesive property with the plastics substrate is poor and the film thickness is thin, so that the sufficient surface hardness can not be obtained.
As for the antistatic resin film mixed with an electro-conductive filler, it is known that the antistatic effect is high. However, in the antistatic resin film mixed with the electro-conductive filler, stability and light transmission properties can not be satisfied at the same time. More specifically, when the electro-conductive filler comprises only tin oxide, stability, shelf life and reproducibility need to be improved. When tin oxide doped with antimony has preferable stability but low light transmission properties. In addition, since the surface of the antistatic resin film mixed with the electro-conductive filler is not flat surface, when it is used in the optical disc substrate, incident beam is scattered, causing the light transmission properties to be lowered and recording and reproducing signals to deteriorate.